Friction stir welding is a solid-state (sub-solidus) joining process typically applied to metallic materials. It involves forcing a spinning, non-consumable tool into and along a joint between two or more workpieces to be permanently joined. A fully consolidated metallurgical joint may be formed between the workpieces as a result of the mechanical stirring and frictional heat introduced into the joint by the rotating, moving (traversing) tool. A conventional friction stir welding tool includes a shoulder and a probe that extends outwardly from the shoulder. During the FSW process, the probe is forced into the workpieces up to the shoulder and travels longitudinally along the joint interface (sometimes referred to as a faying surface) therebetween.
In full penetration welds, the probe provides through-thickness stirring of the joint interface to form a fully consolidated weld nugget or stir zone. The shoulder travels under force on the surface of the workpieces along the top edge of the joint interface, and provides both mechanical heating and containment of the workpiece material at the joint interface (at times referred to as “joint material”) to keep it from escaping the stir zone. More information regarding conventional friction stir welding may be found in U.S. Pat. No. 5,460,317 to Thomas et al.; R. S. Mishra et al., Friction Stir Welding and Processing, 50 MATERIALS SCIENCE AND ENGINEERING R 1-78 (2005); and R. S. Mishra et al. (eds.), FRICTION STIR WELDING AND PROCESSING (2007), the contents of all of which are incorporated by reference in their entireties. Information regarding the conventional joining of different aluminum structural alloys in particular by friction stir welding may be found in P. L. Threadgill et al., Friction Stir Welding of Aluminium Alloys, 54 (no. 2) INTERNATIONAL MATERIALS REVIEWS (2009), the content of which is incorporated by reference in its entirety.
A friction stir welding tool may include an integral probe or, as shown in FIG. 1, a separate probe 100 that is insertable into the shoulder. As shown, the separate probe includes a shank and 102 and a tip 104. The shank may be held in the body of the weld tool with a set screw or bolt, which may be tightened against a whistle-notch 106 located in the shank. The tip is designed to extend outwardly from the shoulder, and it often includes threads 108 that are, in effect, small protruding geometric features. Different screw thread forms have been used in friction stir welding tools including, for example, unified threads, pipe threads, and power screw threads. As shown, the tip includes threads and a set of spiraled flats 110.
The tip 104 of the probe 100 often includes threads 108 as an effective and efficient means for moving material around and along the tip. However, threads tend to break (“shed”) with use during the FSW process, leaving small fragments of the probe in the joint material and rendering the probe less effective. Threaded tool probes are particularly susceptible to thread breakage at the end 112 of the probe tip 104 opposite the shank 102 because partial threads 104a are present at this location. Even when a chamfer is machined into the end of the probe, the threads become partial threads at this location and are prone to breakage (especially during the plunging phase of a typical FSW cycle).
The threads 108 of a tool probe 100 also tend to constrain the joint material to flow around the tool probe in a spiral path, which is imposed on the material by the spiral nature of threads. As such, the joint material is engaged by the strong auguring motion of the threads. Given the relatively high rotational speeds of the probe, threads may be stripped similar to when the threads of a nut get stripped out by over-tightening. Shavings of parent material produced in this way must be recombined to form a solid joint in the wake of the tool, especially in high-strength aluminum alloys. The result is a relatively large content of interfaces in the recombined material behind the probe in the stir zone.
Therefore, it may be desirable to have a tool probe designed to more efficiently move material front to back of the probe as it rotates and advances during a FSW process, while providing sufficient heat, pressure and relative translation of the joint material from each workpiece being joined without the use of threads or serrations.